1. Field of the Invention
The present invention relates to a position finding system of the type in which radio signals are transmitted from a plurality of transmitters to enable the position of mobile receivers to be determined.
2. Description of the Prior Art
Many position finding systems, especially navigation systems are known which operate by determining the relative times of arrival or relative phases of signals received from transmitter pairs, using at least three transmitters. As is well known, such systems establish sets of hyperbolae, one set for each transmitter pair and, providing steps are taken to resolve the ambiguity as to the "lane" in which the mobile receiver is, it is possible to obtain a fix at the intersection of a hyperbola pertaining to one transmitter pair with a hyperbola pertaining to another transmitter pair. As is also well known, a mobile receiver can operate in such a system using the signals from just two transmitters providing the mobile receiver is capable of measuring absolute phase, i.e. the receiver incorporates an atomic time standard, such as a local oscillator circuit which is synchronised when the vehicle is stationary at a known geodetic position
In practice, local conditions may require signals to be used from more than the theoretical minimum number of transmitters (two with an atomic time standard, three without). The availability of redundant information enables accuracy to be increased as mentioned in U.S. Pat. No. 3,419,865 in the context of an inverse system using fixed receivers and a mobile transmitter. GB Pat. No. 1 321 730 discusses other reasons for employing extra transmitters in an Omega system.
There are many variants of hyperbolic position finding systems including Loran, Decca Navigator and Omega. The present invention is applicable to any of these known systems and is not concerned with the particular nature of the signals transmitted or with the way in which the receiver extracts the position information from the received signals. These matters are very well known in the art at least for the three specific systems mentioned above. The invention may nevertheless be applied in particular to the Omega system for the practical reason that Omega provides worldwide coverage from eight transmitters in Hawaii, North America, Argentina, Norway, Liberia, La Reunion, Japan, and Australia. The principles on which Omega is based are described in U.S. Pat. No. 2,844,816.
The basic frequency of the Omega network is very low (10.2 kHz) and the waves propagate in waveguide mode between the ionosphere and the surface of the earth. The propagation of the VLF signals is affected by such factors as the height of the ionosphere and the surface conductivity and a raw Omega fix has a large margin of error, say 7.times.1852 m (1852 m=1 nautical mile). Although such a fix may be of value on the high seas the need for better accuracy is apparent and some improvement is possible using published tables of corrections which are related to locality and time of day (since the height of the ionosphere varies very much in dependence upon the latter factor). By such means an accuracy in the order of 2 to 5.times. 1852 m with a 95% probability may be obtained. Even so the system remains suited only to high-seas navigation.
A substantial increase in the accuracy of Omega is obtained by Differential Omega which is in use for example, in French and North West African coastal waters and can reduce the margin or error to the order of hundreds of meters. Each coastal area is served by a fixed receiving station which repeatedly takes fixes, compares these with its known position and transmits correcting data used by the mobile receivers in its area to correct their fixes on the assumption that the error is a simple offset, expressible as an offset in latitude and an offset in longitude measured at the fixed receiving station, which affects all fixes uniformly over the whole area. This is partially true and the accuracy of 95% probability in Differential Omega is 0.2.times.1852 m up to 20.times.1852 m, 0.3.times.1852 m up to 60.times.1852 m, 0.5.times.1852 m up to 300.times.1852 m and 0.9.times.1852 m up to 500.times.1852 m. In practice the correction may be introduced before calculating latitude and longitude, by phase adjustment within the receiver.
The present invention is concerned with an even more severe problem in accuracy. There has long existed a need for a position finding system which will enable a fleet operator to control a fleet of vehicles effectively on land. Typical examples are police cars and ambulances. There is at present no known system usable for such fleets of vehicles which is both technically acceptable and sufficiently inexpensive to be installed in large numbers of vehicles. There are considerable attractions to use of Omega in that the transmissions are there, anywhere in the world and Omega receivers are available at reasonable cost. However, even with Differential Omega the margin of error over sea is about 300 m at a range of 50 km and increases with range; over land further deterioration in accuracy occurs due to local propagation variations and distortions, for example due to variations in conductivity of the soil and diffraction by topographical and man-made objects.
The accuracy required for locating land vehicles depends on the operator, the environment and the area to be covered. According to a recent report by the U.S. Department of Transport most of the requirements lie in the range 75-300 m.
One approach to increasing accuracy, U.S. Pat. No. 4,232,317, is to record hyperbolic coordinate measurements with a navigation receiver at a large number of points over the operational area and to store these in a computer. The coordinates measured by the vehicle received are transmitted to base and compared with the stored values to determine the nearest value and hence the nearest point to the vehicle. Such a system does not take account of time varying effects, which in Omega are large, and would be prohibitively expensive to apply over a large area. It is therefore an object of the invention to provide an improved system which is capable of an accuracy of, say around 100 m of error and can be applied over a large area at a minimal cost. It is emphasised that the invention is applicable to hyperbolic position finding systems in general; it is not restricted to Omega although Omega is the preferred implementation.